Conventional techniques relating to small volume droplet transfer, to which the present invention relates, will be described below. In this specification, “a small volume droplet” refers not to a shape of a liquid droplet, but to a sate of a liquid droplet clearly separated from other liquids.
There are various techniques for transferring small volume droplets. They include, for example, capillary systems and microfluidic MEMS (mechanical and electrical microsystems) In such systems, a surrounded micro-flow path is used, and a droplet to be transferred is placed between gases allowing, for example, a pressure difference between the gases to be used as the power to transfer the droplet. There are also similar systems in which a droplet to be transferred is placed between liquids, instead of gases, which do not mix with the droplet, and the droplet is transferred as in the systems using gases.
Techniques used to transfer a small volume droplet on a plane include EWOD (Electrowetting on Dielectric) and dielectrophoresis techniques. In these techniques, a droplet is placed on a plane or between two planes, and voltage is applied to electrodes installed on or inside the plane or one of the two planes thereby remotely applying a force to move the droplet.
In a technique used to transfer a droplet in air, a droplet is formed and made to fly using a piezoelectric device. A flow site meter used for cell fractionation is an example of a device using the technique. In such a flow site meter, an electric charge is applied to a small volume droplet formed by using a piezoelectric device, the magnitude of the electric charge being dependent on a cell enclosed in the droplet, and the flight curve of the droplet is distorted using a resultant magnetic field so that a required volume of fraction can be sampled. There is also an example application in which droplets are made to fly in two crossing directions for droplet merging and mixing.
There is a still another method of droplet transfer in which a small volume droplet is transferred being placed on a different member. In an example application of such a method (disclosed in JP-A No. 271185/1999), a liquid is scooped with a hydrophobic-surfaced ring included in a transfer member, and the liquid scooped and spherically formed by its surface tension is transferred being held on the ring of the transfer member. In a still another method (disclosed in JP-A No. 509274/2002), a ring-shaped member included in a device for depositing a liquid dot on a surface holds liquid formed like a film, the liquid is pricked with a pin, and the liquid dot formed on the pin is deposited on a surface.